News inks are prepared by mechanically dispersing carbon black in mineral oil. The oil serves as a vehicle for the carbon particles. The Theological properties of the dispersion are determined by the dispersion properties, and the concentration and the interaction of the oil and the carbon black. They are of great importance in the ultimate use of the ink.
When conventional furnace carbon blacks are dispersed in ordinary mineral oils to a concentration required for good printing, which usually is in the range of 10 to 20% by weight, the carbon particles tend to chain together to form a dispersion having undesirable thixotropic properties. This can be overcome by adding more oil to the dispersion, but it would ordinarily make the pigmentation too weak. It is customary, instead, to give the ink the required "length" by making use of a mineral oil containing resinous compounds or substances or asphaltic still bottoms rich in polar compounds, which will coat the carbon particles and reduce their tendency to chain together.
It is a common practice to use pigment concentrates in current manufacturing of printing inks. Thus, color inks are produced by the appropriate dilution of flushes containing between 25-40 wt. % of pigment. Similarly, black inks are made by letting down suitable bases containing 20 to 35 wt. % of carbon black.
The production of such concentrates allows for more efficient use of manufacturing equipment. It also offers an economical advantage when these concentrates are shipped to blending operations located out of town, or exported overseas. This is because the shipping costs per pound of finished ink can be proportionally reduced. Thus, there is a trend to produce concentrates with as high a pigment content as is practically feasible. However, the viscosity and consistency of concentrates increases very rapidly with an increase in pigment content. Although the application of heat can often assist the processing, it does not always lead to the most desirable results. For instance, during production of carbon black dispersions, generated heat during processing results in a highly structured, short mass which impairs the dispersion process and, thus, limits pigment loading. Further limitations in pigment content of concentrates are also imposed by the available production equipment for processing, and material handling considerations.
In the current state-of-the-art of news ink manufacturing, it is customary to produce dispersions of carbon black in a semi-continuous process consisting of premixing, shot milling, and letdown steps. The premix is conducted in high intensity mixing equipment at a relatively high pigment loading. The parameters of this step are designed to break up and deagglomerate pelletized carbon black. Resinous or polymeric additives are usually used to assist the wetting and dispersion of pigment. The most common additives are asphaltic still bottoms, gilsonite and hydrocarbon resins. These materials, although quite useful, offer limited help since they are effective only at a relatively low processing temperature. For example, at a temperature around 100.degree. C., dispersion of carbon black exhibited poor flow characteristics, limiting the mass flow necessary for efficient mixing and shot milling. Conversely, at ambient temperature, they produce an excessive flow and require the use of rheological additives such as bentonite clays which are expensive and contribute to high cost. This phenomenon is caused by the solubility/temperature equilibrium of these additives in mineral oils. At ambient conditions, their affinity for carbon black dominates their behavior. Thus, they adsorb at the surface of carbon black, producing a relatively stable dispersion with a low, low shear viscosity. At elevated temperatures, due to increased solubility, they desorb and migrate into the bulk of the mineral oil, thus exposing the surface of particles to flocculation and the formation of a network of chains. This flocculated state produces structure and short body, impairing mixing and compounding.
U.S. Pat. No. 4,747,964, describes the use of alkenyl succinimides as components of dispersing additive compositions for lubricating oils. E. S. Forbes and E. L. Newstadter's publication in Tribology, (April 1972, pp. 72-77) elucidates the mechanics of the action of polyisobutylene succinimide in stabilizing the suspension of carbonaceous matter and prevent the formation of sludge in motor lubricating oils. Polyisobutylene succinimide was therefore described as exhibiting good adsorption affinity towards particulates of carbon black. The rational is that long chains of surfactant extended from the surface of particles sterically hinder flocculation and the formation of agglomerates.
Despite the separate use of either sodium hydroxide or alkenyl succinimide, the desired high pigment loading of up to 40% by weight carbon black concentration could not be achieved while maintaining good rheological properties. Furthermore, while the use of each of these compounds separately improves the overall characteristics of the carbon black concentrates in mineral oil and in the final lithographic ink, additional improvement are still required. The present invention makes use of an alkali metal hydroxide in combination with an alkenyl succinimide in order to achieve surprising synergistic effects with regard to improving the Theological properties of ink concentrates and allowing for an increase in the concentration of black carbon in these concentrates.